I'll be honest with you here: Mirage Island was such an insignificant thing in the third generation that I really don't think it should make a return. u___u Admittedly, I was really excited for it back then, and then I managed to stumble upon it, and was kind of disappointed. I mean, all it was really was an island full of Wynaut(and a few other things, maybe some rare berries?), so yeah, ever since then, I never really had a thing for it.

Edit: Whoops, this is why I should read things completely. Anyway...

It would be nice to see a return of some sort of disappearing/re-appearing island, but I don't have my hopes up generally because Mirage Island kind of let me down. :x